This invention relates to improvements in a spindle and Z-axis unit for drilling machines and, more particularly, to a spindle unit whose moving weight or mass is substantially reduced to minimize driving power and noise while, at the same time, permitting the drilling of larger diameter, high quality holes and increasing acceleration control.
FIG. 1 shows a conventional spindle unit designated generally by the numeral 14 widely used for drilling machines which drill holes in printed circuit boards (PCBs). A hollow rotor shaft 101 includes a built-in copper core 110, a thrust flange 103, and a tapered collet 24 for chucking a drill bit 25. The rotor shaft 101 is supported by a radial air bearing 102 and by an axial thrust air bearing 105 located inside of spindle body or housing 104. A selectively energized motor coil 106 surrounding the core 110 drives the rotor shaft 101 for a drilling operation. An arrow 111 shows a direction of a supply of air to the air bearings 102,105 through an inlet port on the side of a diaphragm body at the top of the spindle housing 104. The direction of an air supply for a collet actuating diaphragm 107 mounted in a cavity in the diaphragm body 108 is designated by arrow 112. In an automatic tool change (ATC) process, the diaphragm 107 when actuated by the air supply in direction 112 pushes the collet 24 downwardly axially relative to the rotor shaft 101 in the direction of arrow 113 via a push rod 114 located concentrically within the rotor shaft 101. This downward movement opens the jaws of the collet 24 to allow an ATC operation for the drill bit 25.
FIG. 2 and FIG. 3 show a conventional spindle of the type shown in FIG. 1 mounted on a conventional Z-axis unit. A drive motor 7 is mounted on a unit base 6 to drive a screw shaft 11 supported by ball bearings 9 also located on the unit base 6. Screw shaft 11 drives a screw nut 12 to which a spindle saddle 13 is mounted for movement in an axial direction (the Z-axis) of the spindle 14. The spindle saddle 13 can be supported and guided by a known linear guide mechanism. The spindle saddle 13 has the conventional spindle unit 14 fitted therein and reciprocates the spindle unit 14 in the Z-axis direction, and also reciprocates an axially movable pressure foot 20 with a chip evacuation system 39 supported, axially guided and moved 15 by brackets 15 mounted on the spindle saddle 13, a pair of air cylinders 16, swivel joints 17, and shafts 19 supported by bearings 18 located at both sides of the spindle unit 14.
In the typical drilling process, the spindle saddle 13 is caused to move downwardly by actuation of the screw nut 12, and the pressure foot 20 first contacts a surface of a PCB 22 to be drilled. The pressure foot 20 clamps the PCB 22 by an actuation force of the air cylinders 16. The spindle unit 14 is then advanced in the Z-direction through an opening in the pressure foot 20 and drills the PCB 22 via the drill bit 25 which is rotated when the motor coil 106 is energized to rotate the rotor shaft 101. As soon as the tip of the drill bit 25 reaches a specified depth (the down limit), the pressure foot 20 and the spindle saddle 13 retract to a resting position (the up limit), and a tooling table 21 upon which the PCB 22 is secured and a spindle carriage (not shown) move to the next drilling position to repeat the pressure foot clamping and drilling process.
In the above described Z-axis unit, the total weight of the various moving parts exceeds 15 kg. Most of the weight is attributable to the spindle saddle 13 and the spindle body 104. Accordingly, high acceleration control which is necessary for high speed positioning cannot be attained satisfactorily and undesired noise is produced by excessive G forces. Reductions in the size of the drive motor and other spindle mechanisms have not been achievable up to now as a result.
FIG. 4 shows another conventional form of spindle unit designated generally by the numeral 14' which uses a centrifugal collet 115 at the end of a rotor shaft 101' instead of a diaphragm and tapered collet of the type shown in FIG. 1 to reduce the moving weight of the unit for higher acceleration control and noise reduction. Parts similar to those shown in FIG. 1 are designated with the same numerals but are primed. The rotor shaft 101' includes a built-in copper core 110' which is slightly longer than the core 110 shown in FIG. 1 for permitting axial movement. Radial air bearings 102' are provided along the axial length of the rotor shaft 101', and a thrust flange 103' is integral with the rotor shaft 101'. A thrust air bearing 105' in a thrust bearing assembly 38 at the top of the unit 14' supports the flange 103'. A selectively energized motor coil 106' surrounds the core 110'. A supply of pressurized air is provided to the radial bearings 102' in the direction shown by arrow 111' through a port (unnumbered) in the spindle housing or body 104'. Unlike the spindle unit 14 in FIG. 1, however, the thrust air bearing assembly 38 in the spindle unit 14' of FIG. 4 is supported and guided in the axial or Z-axis direction by a special radial air bearing 117' and an air groove 118' located on the radial periphery of the thrust bearing assembly 38 instead of through a relatively massive spindle saddle and the spindle housing. An arrow 116' shows the direction of a supply of air for the thrust air bearing 105' through a port in the assembly 38. A rod 32 fixedly mounted on the thrust bearing assembly 38 moves the drill bit 25 in the axial or Z-direction designated by double-headed arrow 119 via the thrust air bearing assembly 38, the rotor shaft 101' and the centrifugal collet 115'. With such an arrangement, the moving weight of the spindle unit 14' is reduced to about 1.5 kg. However the centrifugal collet 115' in this form of spindle has a chucking or clamping force which is unacceptably small at low speed ranges from about 15 to 30 Krmp. It is necessary in order to achieve high hole quality (i.e. no roughness, no smearing) for drilling hole sizes of 0.08" to 0.25" to maintain an adequate chucking force at the low speed range. As a result, although the total moving weight of the spindle unit 14' can be decreased with a centrifugal collet 115' of the aforementioned type so as to increase acceleration control and reduce noise, drill bit sizes are limited to 0.08" or lower to ensure adequate chucking force.
FIG. 4E shows details of the centrifugal collet mechanism used in the spindle shown in FIG. 4. In particular, a front end of a rotor 101' is supported in radial air bearing 102' so as to rotate around the same axis as the spindle body 104'. A depression 3a is formed in the shaft 101' and is open toward the front of the rotor with a suitable depth in the axial direction. A guide portion 6a provided with a central fitting hole 5a to which a tool 25 fits is inserted into the interior of the depression 3a. At the bottom of the depression 3a, a cylindrical guide portion 8a having a central fitting hole in which the tool 25 fits is inserted so as to extend axially. The guide portion 6a with the central fitting hole 5a, the cylindrical guide portion 8a and the central bottom hole 7a formed on the bottom of the rotor 101' are adapted to be concentrically fitted with an allowable clearance of several microns.
A centrifugal piece 13a provided with a fitting hole 12a to which the guide portion 8a is applied or fitted and another fitting hole 9a through which the tool 25 is inserted is situated in the hole 3a with predetermined clearances or gaps 10a and 11a to the inner surface of the depression 3a and an outer surface of the tool 25. The centrifugal piece 13a is statically held within the depression 3a so as to keep the predetermined clearance 10a and 11a by means of an O-ring 14a situated as shown around the guide portion 8a. The fitting hole 9a of the centrifugal piece 13a through which the tool 25 is inserted is designed to have substantially the same fitting allowance as that of the central fitting hole 5a and the central bottom hole 7a. The tool 25 is prevented from dropping out of these holes 5a, 7a and 9a when it is static or stable by means of another O-ring 15a fitted around the cylindrical body of the tool 25.
When the rotor 101' rotates during a drilling operation of the spindle, the centrifugal piece 13a eccentrically rotates together with the rotary motion of the rotor 101' resulting in transformation of the rotary center of gravity due to eccentric rotation. As a result, a centrifugal force is generated in the cylindrical guide portion 8a and the tool 25, and a torque or a force couple is generated, so that the tool 25 is firmly held in place by a reaction force of the fitting faces of the fitting holes 5a and 7a, and friction coefficients of the fitting faces. Consequently, when the rotor shaft rotates at a speed less than 30 Krpm, it is difficult to obtain sufficient centrifugal force of the tool because the collet slips on the tool holding surface.